1. Field of the Invention
The present invention relates generally to Coriolis mass flow meters, and more particularly to a device comprised of two elongated straight tube segments extending in parallel with each other and joined together in such fashion that an inlet portion of a loop is formed by one tube segment and an outlet portion is formed by the other tube segment and such inlet and outlet portions form parallel Coriolis responsive legs of a parallel tube Coriolis mass flow sensor.
2. Discussion of the Prior Art
Straight tube Coriolis mass flow sensing apparatus has long been known in the prior art. For example, the U.S. Pat. Nos. to W. C. Wiley et al, No. 3,080,750 and A. J. Sipin, No. 3,329,019 provide early disclosure of single tube, straight tube Coriolis devices and provide technical arguments in explanation of the operation thereof. Briefly stated, in both devices the tube ends are fixed in position and the mid-point is driven is oscillatory fashion with a measure of the Coriolis induced forces being used to indicate mass flow rate. In Sipin, Coriolis induced phase difference is measured at points on either side of the tube mid-section and such phase difference (a function of Coriolis forces) is used to indicate mass flow rate.
A later disclosure of a straight tube device in which mass flow rate is measured by sampling Coriolis induced torque as the tube passes through a mid-line of oscillation is disclosed in U.S. Pat. No. 4,109,524 issued to James E. Smith.
It was also recognized by Roth, U.S. Pat. No. 2,865,201 and Sipin, U.S. Pat. No. 3,355,944 that a Coriolis mass flow sensor could be made by folding a tube into a U-shaped or looped form. The U-shaped design was modified in Smith U.S. Pat. Nos. 4,187,721 and 4,422,338 by adding a counterbalancing spring arm to the drive mechanism.
In Cox et al, U.S. Pat. No. 4,127,028 disclosed that by using a pair of U-tubes disposed in parallel and oscillating them in opposite directions, one could make differential measurements of Coriolis induced phase differences and at the same time, achieve the benefits of a "tuning fork" design having more favorable density response characteristics.
Further improvement in which generally one or more helically wound tubes are utilized are disclosed in the U.S. patent applications of Erik Dahlin, Ser. No. 775,739 filed Sept. 13, 1985, now U.S. Pat. No. 4,711,132 and Erik Dahlin et al, Ser. No. 777,707 filed Sept. 13, 1985, now U.S. Pat. No. 4,660,421 (both of which are assigned to the assignee of the present invention). Another flow meter apparatus in which two U-tubes are joined together by a manifold structure to split the flow and provide fluid flow through each conduit is disclosed in the U.S. patent of James E. Smith et al, U.S. Pat. No. 4,491,025. A flow meter using an S-shaped tube is disclosed in the U.S. patent to A. J. Sipin U.S. Pat. No. 4,559,833. The latter patent also discloses the use of two such S-shaped tubes disposed in parallel and in combination with appropriate flow splitters.
Although the above mentioned prior art devices are quite suitable for certain applications, one of the features of the split flow devices which adds to the complexity and cost thereof is the manifold structure that is required in order to split the flow into separate flow paths which can then be oscillated relative to each other to accomplish Coriolis mass flow detection. In addition to the physical complexity of the manifold structure, it also has the disadvantage that it requires the use of metallic junctions which are not suitable for applications in the food and pharmaceutical industry wherein very high degrees of contamination-free joint and conduit forming wall structures ie., ultra smooth surfaces, are required. Another problem encountered in split flow devices is that it can not be assured that phase separation will not occur in the fluid and result in a compromise of, or even loss of, the tuning fork effect normally expected in such devices.